WO2016097191A1

WO2016097191A1 - Device for transporting and controlling light pulses for lensless endo- microscopic imaging

Info

Publication number: WO2016097191A1
Application number: PCT/EP2015/080312
Authority: WO
Inventors: Esben ANDRESEN; Hervé RIGNEAULT
Original assignee: Centre National De La Recherche Scientifique (Cnrs); Université D'aix-Marseille
Priority date: 2014-12-18
Filing date: 2015-12-17
Publication date: 2016-06-23
Also published as: FR3030956B1; FR3030956A1; US20180011309A1; JP6720183B2; EP3234666A1; US10571678B2; JP2018502638A

Abstract

According to one aspect, the invention relates to a device for transporting and controlling light pulses for lensless endo-microscopic imaging and comprises: a bundle of N monomode optical fibres (Fi) arranged in a given pattern, each monomode optical fibre being characterized by a relative group delay value (Ax) defined relative to the trip time of a pulse propagating in a reference monomode optical fibre (F ₀) of the bundle of fibres (40); an optical device (50) for controlling group speed, comprising a given number M of half-wave plates (P _j) that are characterized by a given delay (8t _j); a first spatial light modulator (51) suitable for forming from an incident light beam a number N of elementary light beams (B _i) each of which is intended to enter into one of said optical fibres, each elementary beam being intended to pass into a given half-wave plate such that the sum of the delay introduced by said half-wave plate and the relative group delay of the optical fibre intended to receive said elementary light beam is minimal in absolute value; a second spatial light modulator (52) suitable for deviating each of the N elementary light beams such that each elementary light beam penetrates into the corresponding optical fibre perpendicularly to the entrance face of the optical fibre.

Description

Light impulse transport and control device for endo-microscopic imaging without lens

STATE OF THE ART Technical field of the invention

The present invention relates to a device for transporting and controlling light pulses for so-called "no lens" endo-microscopic imaging as well as endo-microscopic imaging systems and methods without a lens, and in particular non-linear imaging. It applies in particular to the endoscopic exploration of organs in a living being, human or animal.

State of the art

The developments in endo-microscopic imaging require the use of opto-mechanical devices fimbriated with specificities compared to imaging systems in free space.

On the one hand, the construction of a miniature microscope that would include a light source, focusing optics, and a camera at the distal end (i.e. at the end of the fiber, on the sample side ) a medical endoscope is not possible because of the congestion of all components. In this way, solutions are sought that make it possible to perform imaging at the end of an optical fiber while limiting the bulk at the distal end of the endoscope.

On the other hand, the growing interest in endo-microscopy nonlinear imaging requires working with light pulses with high light intensities, which is not always compatible with the fimbriated systems. Nonlinear imaging techniques include for example bi-photon fluorescence imaging (or TPEF according to the English expression "two-photon excited fluorescence"). This imaging technique is particularly interesting in endo-microscopy because the interaction between light and matter is confined to the focal point, so there is no background signal generated out of the focal point and therefore a spatial resolution three-dimensional is possible allowing an optical sectioning (or "optical sectioning"). TPEF imaging also makes it possible to use an excitation laser Near-infrared wavelength, which penetrates deeper into a diffusing medium such as a biological tissue. Other non-linear processes may be interesting in endoscopy to access additional information; this is the case of the 3-photon or 3PEF fluorescence for "Three-Photon Excited Fluorescence", the second-harmonic generation or SHG for "Second-Harmony Generation", the third-harmonic generation or THG for "Third-Harmonic Generation ", Coherent anti-Stokes Raman scattering or CARS for" Coherent Anti-Stokes Raman Scattering ", stimulated Raman scattering or SRS for" Stimulated Raman Scattering ".

There are several approaches for performing imaging at the end of optical fiber while limiting the bulk at the distal end of the endoscope, these techniques being more or less well suited to nonlinear imaging.

A first approach (described for example Rivera et al., "Compact and Flexible Multiphoton Endoscopic Raster Scanning Capable of Imaging Unstained Tissue", Proc Nat Nat Sci USA, 108, 17598 (2011)) is to vibrate the part distal of a monomode optical fiber, for example using piezoelectric shims, the end of the optical fiber being imaged in the sample using a micro-optics. The optical fiber makes it possible to deliver the light into the sample and to collect the signal coming from the sample, this signal being derived for example from reflection, fluorescence or a non-linear interaction in the sample. However, the presence of a piezoelectric scanner at the end of the fiber limits the diameter below which the distal portion of the endoscope can be miniaturized (typically of the order of 3 mm); Moreover, the control of the imaging plane along the optical axis is complex to implement. Finally, this approach is limited for non-linear imaging that requires the use of ultra-short pulses (typically less than one picosecond). Indeed, the standard optical fibers have a strong dispersion that is difficult to pre-compensate and they are subject to non-linear effects that affect the spectral and temporal profiles of the light pulse delivered at the end of the fiber.

It is also known to use a packet of optical fibers, or "bundle" (which can comprise up to 30000 fibers) and to illuminate the fibers one by one thanks to a scanner located in the proximal portion (see for example Knittel et al. ., "Endoscope-compatible confocal microscope using a gradient index-lens system", Common Opt. 188, 267 (2001)). The optical fibers are imaged in the sample by a micro-optics located in the distal portion and a sequential scanning of the optical fibers of the fiber bundle makes it possible to obtain an image in the same way as in a confocal microscope. However, as in the previously described approach, all the light flows through the core of a single optical fiber and the peak peak power is limited because of the intrinsic nonlinear effects present in the optical fibers of the fiber bundle. It is therefore difficult to make non-linear imaging that requires ultra-short pulses, and therefore high peak powers.

A third approach, termed "endoscopy without a lens", and described for example in Cizmar et al. Exploiting Multimode Waveguides for Pure Flu-based Imaging, Nat. Common. 3, 1027 (2012), is based on the use of a multimode optical fiber, or MMF according to the abbreviation of the Anglo-Saxon expression "Multi-Mode Fiber". The MMF optical fiber is illuminated with a coherent source. On the proximal side (that is, at the optical fiber input, on the opposite side of the sample) of the optical fiber MMF, a Spatial Light Modulator (SLM) allows play on the modes of propagation of the fiber so that the coherent addition of these modes makes it possible to generate all intensity figures at the end of the MMF fiber. In one embodiment, an attempt is made to produce a focal point at the end of the MMF fiber and to scan the sample to obtain an image as would be done in a conventional confocal microscopy setup. This technique, extremely powerful because of the deterministic nature of the transmission matrix of the fiber which connects a field entering the proximal portion of the fiber with a distal outgoing field (and vice versa), eliminates all optics the distal side of the multimode fiber and thereby reduce the bulk. However, the transmission matrix of the fiber is complex and highly dependent on the curvature of the optical fiber MMF. Endo-microscopic imaging using an MMF optical fiber is therefore extremely sensitive to any movement of the fiber. Moreover, because of the multimode nature, a short pulse in the proximal part is greatly elongated in the distal portion, which limits the possibilities of application to nonlinear imaging.

In parallel with technologies based on the use of multimode fibers, a "lensless" type of technology has developed based on the use of a single-mode optical fiber packet (see, for example, French et al US Patent No. 8,585,587). ). According to the technique described, a wavefront spatial modulator (SLM) arranged on the proximal side of the monomode optical fiber packet makes it possible to control the wavefront emitted by a light source at the distal end of the fiber packet. Because there is only one mode and therefore no mode dispersion in monomode optical fibers and it is possible to compensate for chromatic dispersion effects in a global manner, the use of a packet of fibers Single-mode optics allows, compared to multimode fibers, the propagation of short pulses. In addition, the possibility of distributing light energy on all fibers allows the propagation of high intensity pulses, opening the way for nonlinear endoscopic imaging. Various publications have described variants of endo-micro-scopy without lenses based on the use of a packet of single-mode optical fibers and more specifically of a multi-core fiber or "MCF" according to the English expression "Multi -Core fiber. For example, it is shown how one can access, in the distal portion of the packet of optical fibers, a very fast scanning of the focusing point, by printing by means of a galvanometric device a variable angle of the input wavefront. of the SLM (see, for example, ER Andresen et al., "Toward endoscopes with no distal optics: video-rate scanning microscopy through a fiber bundle", Opt Lett., Vol 38, No. 5 (2013)). In ER Andresen et al. ("Two-photon lensless endoscope", Opt.Express 21, 20713 (2013)) the authors have demonstrated the experimental feasibility of a non-linear bi-photonic imaging system (TPEF) in endoscopy without lens.

FIG. 1A schematically illustrates an endo-microscopic imaging system without a lens 100 as described in the prior art and applied in particular to nonlinear imaging. The imaging system 100 generally comprises a transmission source 10 for transmitting an incident beam formed by Io pulses in the case of the application to nonlinear imaging. The system 100 further comprises a detection channel comprising an objective 21 and a detector 20. The detection path is separated from the emission path by a separating blade 22. The imaging system 100 also comprises a transport device and Io pulse control for illuminating a remote analysis object 101. The transport and control device comprises a packet of monomode optical fibers 40 whose input and output faces 42 are shown enlarged in FIG. 1A, and a wavefront spatial modulator ("SLM"). ) Arranged at the proximal end of the fiber bundle 40 and for controlling the wavefront of the beam emitted by the source 10. The spatial light modulator makes it possible to print on the incoming wavefront having a function of phase Φο defined phase shifts Φι (ί) for each elementary beam Bi intended to enter an optical fiber Fi of the fiber packet 40. The phase function Φι (ί) may be such that, for example, after propagation in the packet of optical fibers, the wave comes out with a parabolic phase Φ ₂ (ί). This parabolic phase allows the beam to focus distally on the analysis object 101 while there is no physical lens present; this is the origin of the terminology "endoscope without lens". Moreover, it is possible thanks to the spatial light modulator to compensate for phase shifts introduced by each of the optical fibers Fi.

However, applicants have demonstrated that in a non-linear imaging mode, i.e. when ultra-short pulses are sent into the optical fiber packet single-mode, typically pulses of shorter duration than the picosecond, the group delays of the pulses traveling in the different optical fibers may generate a loss of light intensity on the sample. The electromagnetic field E ^ (t) describing an elementary beam Bi formed by pulses at the distal end of the fiber bundle can be expressed as:

Where s (t) = E ^ (t) is the electromagnetic field describing the elementary beam Bo propagating in the fiber Fo taken as reference, φ (ί) represents the phase term and Δχ (ί) is the relative group delay defined with respect to the travel time of the elementary beam B ₀ in the reference fiber Fo.

As illustrated in FIG. 1B, where only the device for transporting and controlling the incident beam is represented, the group velocities Xi (i) of the pulses forming the elementary beams Bi at the output of the SLM 30 and incident in the optical fibers Fi of the fiber pack 40 are constant. In other words, relative group delays are zero or almost zero. On the other hand, at the distal output of the fiber packet, variable group speeds are observed in the different elementary beams described by the function X ₂ (i) and resulting in non-zero relative group delays Δχ (ί). These group delays are introduced by each of the monomode optical fibers Fi and result from intrinsic inhomogeneities inevitably appearing during the manufacture of the fiber as well as from the inhomogeneities induced by stresses during deformation and / or during a movement of the fiber. . The distal output of the fiber bundle 40 results in a temporal widening of the focused pulse on the analysis object 101, this enlargement being accompanied by a decrease in the peak light intensity and consequently a decrease of the signal resulting from the nonlinear process.

The present invention provides devices and methods for transporting and controlling light pulses in a so-called "lensless" endo-microscopic imaging system that enable the control of pulse group delay delays in monomode optical fibers of the packet. fiber. The devices and methods described in this description make it possible to control the duration of the pulses at the distal end of the fiber packet and thus to access non-linear imaging applications which require the transmission of ultra-short pulses, typically less than the picosecond. SUMMARY OF THE INVENTION

According to a first aspect, one or more exemplary embodiments relate to a device for transporting and controlling light pulses with at least one wavelength for endo-microscopic imaging without a lens. The device comprises a packet of N monomode optical fibers arranged in a given pattern, for receiving a light beam formed of pulses at a proximal end and for emitting a light beam at a distal end, each monomode optical fiber being characterized by a value relative group delay defined with respect to the travel time of a pulse propagating in a monomode optical reference fiber of the fiber packet.

The device for transporting and controlling light pulses further comprises an optical device for controlling the group speed or more precisely an optical device for controlling group delays, arranged on the side of the proximal end of the packet of optical fibers. and including:

A given number M of delay blades, each blade allowing the introduction of a given delay;

A first spatial light modulator adapted to form, from one or more incident light beams, a number N of elementary light beams intended to each enter into one of said optical fibers, each elementary beam being intended to pass through a given delay plate such as that the sum of the delay introduced by said delay plate and the relative group delay of the optical fiber intended to receive said elementary light beam is minimal in absolute value;

A second spatial light modulator adapted to introduce on each of the N elementary light beams a deflection such that each elementary light beam enters the corresponding optical fiber perpendicularly to the input face of the optical fiber.

The packet of N monomode optical fibers may be formed of a set of monomode optical fibers, typically from one hundred to several tens of thousands of fibers, collected in the form of a bundle of fibers, periodically or aperiodically, or may be formed a multi-core fiber having a set of single-mode cores, preferably at least one hundred, arranged periodically or aperiodically.

Whether it is a set of single-mode optical fibers assembled in a bundle or a multi-core fiber, we will look for a packet of monomode optical fibers having the lowest possible coupling, advantageously less than -20 dB / m. The M delay plates are advantageously distributed in a plane. The number M of delay plates can be between 1 and a few tens, advantageously between 2 and 20, but in any case, it is much smaller than the number N of single-mode optical fibers in the fiber packet.

Thus, the transport and control device according to the present description makes it possible to minimize the standard deviation of the set of values formed by the group delays of the pulses in the fibers, whatever the fiber packet used and even if the bundle of fibers is displaced or deformed; this is made possible by simply programming each of the spatial light modulators to form elementary beams for entering each of the fibers of the fiber bundle and controlling their movements to pass through the appropriate delay blade.

The transport and control device according to the present description can also allow, by programming one and / or the other of the spatial light modulators, the application of a phase shift on each of the elementary beams, allowing to register at the distal end of the fiber packet a determined phase function and / or correcting the phase variations introduced by each of the fibers of the fiber bundle.

The transport and control device according to the present description can also allow the transport and control of beams formed of pulses of different wavelengths, by programming the first spatial light modulator to ensure the distribution of the elementary light beams. formed of pulses at different wavelengths in subsets of fibers distinct from the fiber bundle.

Spatial light modulators may include segmented or membrane-deformable mirrors (operating in reflection) or liquid crystal matrices operating in reflection or transmission.

Thus, the optical device for controlling the group speed may comprise elements operating in reflection and / or transmission, but a reflection arrangement has the advantage of having more choice on the technology of spatial light modulators.

According to one or more exemplary embodiments, the optical device for controlling the group speed comprises a first objective and a second objective forming an optical assembly with an intermediate focal plane; the delay plates are arranged in the intermediate focal plane of the optical assembly; the first spatial light modulator is in a focal plane object of the first objective; and the second spatial light modulator is in an image focal plane of the second lens. According to one or more exemplary embodiments, the optical device for controlling the group speed comprises an objective; the delay plates are arranged in a plane situated upstream of the first spatial light modulator and are adapted to form, from an incident beam formed by pulses, M light beams, each light beam being formed of pulses characterized by a given group delay; the first spatial light modulator is arranged in the object focal plane of the objective and is intended to receive said M light beams; the second spatial light modulator is in a focal plane image of the lens

For example, the first spatial light modulator is formed of M zones, on which computer-generated holograms are formed, each hologram being adapted to receive one of said light beams formed of pulses characterized by a given group delay.

According to a second aspect, one or more exemplary embodiments relate to an endo-microscopic imaging system comprising a source of light pulses, a device for transporting and controlling the pulses emitted by said source according to the first aspect and a detection channel. light for passing through the monomode optical fiber packet from its distal end to its proximal end.

According to one or more exemplary embodiments, the source of light pulses is a laser source emitting pulses of less than one picosecond duration, advantageously between 100 femtoseconds and 1 picosecond.

According to a third aspect, one or more exemplary embodiments relate to a non-linear endo-microscopic imaging process without a lens by means of a bundle of monomode optical fibers arranged in a given pattern and each characterized by a relative group delay defined by the travel time of an impulse propagating in a monomode optical reference fiber of the fiber packet, the method comprising:

the emission of at least one incident beam formed of pulses at a given wavelength on a first spatial light modulator arranged in the object focal plane of a first objective forming, with a second objective, an optical assembly with a plane intermediate focal;

forming, by means of the first spatial light modulator from the incident light beam, a number N of elementary light beams intended to each enter into one of said optical fibers, each elementary beam passing through a given delay plate arranged in the plane focal point of the optical assembly, such as the sum of the delay introduced by said delay plate and the delay of relative group of the optical fiber for receiving said elementary light beam is minimal in absolute value;

the introduction by means of a second spatial light modulator arranged in the image focal plane of the second objective of a deflection on each of the N elementary light beams such that each elementary light beam enters the corresponding optical fiber perpendicularly to the face of the light. input of the optical fiber.

According to a fourth aspect, one or more exemplary embodiments relate to a non-linear endo-microscopic imaging process without a lens by means of a bundle of monomode optical fibers arranged in a given pattern and each characterized by a relative group delay defined by the travel time of an impulse propagating in a monomode optical reference fiber of the fiber packet, the method comprising:

the emission of at least one incident beam formed of pulses at a given wavelength and the formation from said incident beam and by means of delay plates of a number M of light beams, each of the M light beam being formed of pulses characterized by a given group delay,

forming by means of a first spatial light modulator arranged in the focal plane object of a first objective and from the M light beams of a number N of elementary light beams intended to each enter into one of said optical fibers, such as the sum of the delay of the light beam from which the introduced elementary light beam is formed and the relative group delay of the optical fiber for receiving said elementary light beam is minimal in absolute value; the introduction by means of a second spatial light modulator arranged in the image focal plane of the objective of a deflection on each of the N elementary light beams such that each elementary light beam enters the corresponding optical fiber perpendicularly to the face input of the optical fiber. Advantageously, the relative group delays of the monomode optical fibers of the fiber packet are characterized at the wavelength of the pulses forming the incident light beam.

According to one or more exemplary embodiments, one and / or the other of the spatial light modulators allows the application of a phase shift on each of the elementary beams, making it possible to register at the distal end of the fiber packet a determined phase function and / or correcting the phase variations introduced by each of the fibers of the fiber bundle.

According to one or more embodiments, particularly for applications in Nonlinear imaging in which pulses are interacted at different wavelengths, the method includes the emission of incident light beams formed of pulses at distinct wavelengths. In this or these exemplary embodiments, the first spatial light modulator further allows the distribution of the elementary light beams in distinct and identified fiber subsets of the fiber bundle, each subset of fibers being intended to receive the bundles of fibers. pulses formed of pulses at a given wavelength.

The endo-microscopic nonlinear imaging methods described in the present description apply to any type of non-linear imaging, and in particular the generation of fluorescence and two-photon auto-fluorescence, the generation of fluorescence and n-photon autofluorescence, second harmonic generation, third harmonic generation, nth-harmonic generation, sum and frequency difference generation, coherent Raman signal generation, transient absorption signal generation, transient index modification.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and characteristics of the invention will appear on reading the description, illustrated by the following figures:

FIGS. 1A and 1B (already described), a block diagram of a so-called "no lens" endoscope based on the use of a single-mode fiber packet and a diagram illustrating the problem of group delay in the fibers in the case ultra-short pulses;

Figures 2A and 2B are diagrams illustrating an example of an endoscopic imaging system without a lens according to the present description;

FIGS. 3A to 3D, figures illustrating an example of multi-core optical fiber and its characterization, for the implementation of an endo-microscopic imaging method without a lens, according to the present description;

FIG. 4, a diagram illustrating an example of delay plates for the implementation of an endo-microscopic imaging method without a lens, according to the present description;

FIGS. 5A and 5B, diagrams respectively illustrating the dispersion of group delays in the multi-core fiber shown in FIG. 3A, before and after implementation of an endo-microscopic imaging method according to the present description;

Figure 6, first experimental results comparing the spatial pattern of the focal point at the output of a multi-core fiber shown in FIG. 3A with or without application of an endo-microscopic imaging method according to the present description;

Figure 7 is a diagram illustrating an example of a "lensless" endo-microscopic imaging system according to another example of the present disclosure;

Figures 8A and 8B are diagrams illustrating examples of a "lensless" endoscopic imaging system according to other examples of the present disclosure;

Figure 9 is a set of diagrams illustrating a method for distally measuring group delays in a single mode fiber packet;

Figure 10 is a set of diagrams illustrating a method for proximal measurement of group delays in a single mode fiber packet.

DETAILED DESCRIPTION

In the figures, the identical elements are indicated by the same references.

FIGS. 2A and 2B schematically illustrate a "lensless" endoscopic imaging system 200 according to the present description as well as the principle of implementation.

The system 200 generally comprises a transmission channel, with a light source 10 for the emission of ultra-short light pulses I ₀ , typically less than the picosecond, for example between 100 femtoseconds and a picosecond, and a suitable detection channel. for detecting light for passing through the monomode optical fiber packet 40 from its distal end to its proximal end. The light detected is, for example, the light resulting from the non-linear process in the sample after excitation. The detection path includes an objective 21 and a detector 20 and is separated from the transmission path by a splitter plate 22, for example a dichroic plate in the case of nonlinear imaging applications in which the wavelength detection (for example 2-photon fluorescence) is different from the emission wavelength.

The system 200 also comprises a device for transporting and controlling the light pulses. According to the present description, the device for transporting and controlling the light pulses comprises an optical device 50 for controlling the group speed, or a group delay control device ("GDC" for "Group Delay Control"), a packet N single-mode optical fibers Fi, referenced 40, and advantageously, an optical system 60 telescope type, to adapt the size of the beam from the optical device for controlling the group speed 50 to the input face 41 of the package 40. On the example of In FIG. 2A, the detection path is represented between the light source 10 and the GDC 50. The detection path could equally well be between the GDC 50 and the fiber packet 40, for example between the GDC 50 and the telescope 60.

The N monomode optical fibers F 1 of the fiber pack 40 are arranged in a given pattern. In the example shown in FIGS. 2A and 2B, the monomode optical fibers Fi are arranged periodically; each fiber Fi forms, for example, the core of a multi-core fiber or "MCF". The fiber optic packet 40 includes an input face 41 located on the proximal side, i.e. on the side intended to receive an incident light flux and an exit face 42 located on the distal side, that is, on the side intended for the emission of an outgoing light beam for the illumination of an analysis object 101.

Each optical fiber Fi of the fiber packet is characterized by a relative group delay Δχ; defined by the difference in time that an elementary beam Bi formed by a light pulse passes through the fiber Fi and the time that an elementary beam formed by the same light pulse passes through a reference fiber Fo arbitrarily chosen in the packet fiber. Related group delays Δχ; thus describe the relative delays of the light pulses propagating in the optical fibers Fi. The characterization of relative group delays can be done by known characterization methods which will be described in more detail later.

According to the present description and as generally illustrated in FIG. 2B, the GDC optical group speed control device (50) is arranged on the proximal side of the monomode optical fiber packet 40 and is intended to reduce, on the distal side of the optical fiber packet, the relative difference between the different elementary beams Bi. Thus, the optical device for controlling the group speed 50 according to the present description is adapted to introduce at each elementary beam Bi, intended to enter a monomode optical fiber Fi of the fiber packet 40, a group delay which will at least partially compensating for the group delay Δχ; characterizing the fiber Fi, so that the relative group delays in the different elementary beams at the output of the fiber packet 40 are close to zero and at least less than half the duration of the pulses intended to propagate in the packet of fibers. As shown in FIG. 2B, the control of the group velocities Xi (i) on the proximal side of the fiber bundle results in a substantially constant distribution of the group velocities X ₂ (i) on the distal side.

FIG. 2A illustrates a first example for the realization of an optical device for controlling the GDC group speed according to the present description. The optical device for controlling the group speed 50 comprises in this example a first objective 53 characterized by a focal length f | and a second lens 54 characterized by a focal length f ₂ . The objectives 53 and 54 are defined by any suitable optical system, for example using lenses and / or mirrors. The first and second lenses 53, 54 are arranged to form an optical assembly with an intermediate focal plane (Σi) coincident with the image focal plane of the first objective 53 and the object focal plane of the second objective 54.

The optical device for controlling the group speed 50 also comprises a given number M of delay plates P _j , advantageously between 2 and 20 blades, spatially distributed in a plane, this plane being in the example of FIG. 2A, the intermediate focal plane (Σi). Each blade is designed to allow the introduction of a delay OT given _j.

The speed control device GDC also comprises a first spatial light modulator 51 adapted to form, from an incident beam formed by pulses I ₀ emitted by the light source 10, a number N of elementary light beams Bi intended to form enter into each of the N optical fibers Fi of the fiber pack 40. In the example of Figure 2A, the first spatial light modulator 51 is in an object focal plane of the first objective 53 and is adapted to write on each beam elementary Bi a deviation such that each elementary beam Bi passes into the appropriate delay plate P _j . P _j appropriate retardation plate that scores OT _j delay such that the sum of the delay introduced by the OT _j waveplate P _j and of the relative group delay Δχ; the optical fiber Fi for receiving said elementary light beam Bi is close to zero regardless of the optical fiber Fi or at least less than half the pulse duration. In practice, the number M of delay plates is much smaller than the number N of monomode optical fibers in the fiber packet 40 (for example a multi-core fiber) and a large number of elementary beams Bi are printed with the same delay. . It then tries to minimize the variance of the histogram of the set of values (OT _j + Δχ;) OT where _j is the delay applied to the elementary beam Bi intended to pass through the fiber Fi characterized by a group delay Δχ ;, as will be illustrated through an example later.

The speed control device 50 according to the present description also comprises a second spatial light modulator 52 adapted to introduce on each of the N elemental light beams Bi a deflection such that each elementary light beam Bi enters the corresponding optical fiber Fi perpendicular to the input face of the optical fiber. In the example of FIG. 2A, the second spatial modulator of The light 52 is in an image focal plane of the second lens 54 and makes it possible to compensate for the deflection introduced on each elementary beam Bi by the first spatial light modulator 51.

In the simplified diagram of FIG. 2A, three elementary beams B _ls B ₂ , B ₃ are thus represented. These beams are formed from a beam incident on the first spatial light modulator 51, the incident beam being formed of pulses I ₀ emitted by the light source 10. The beams Bi and B ₂ intended to enter the optical fibers Fi and F ₂ (not shown) of the packet of fibers 40, characterized by group delays Δχι and Δχ ₂ , are deflected by the first spatial light modulator 51 and focused by the first objective 53 so as to pass through a blade to delay Pi characterized by a delay δίι while the beam B ₃ intended to enter the optical fiber F ₃ (not shown) of the fiber packet 40, characterized by a group delay Δχ ₃ , is deflected by the first spatial light modulator 51 and focused by the first objective 53 to cross a delay plate characterized by a delay ôt ₂ . The elementary beams B _ls B ₂ , B ₃ are then sent by means of the second lens 54 to the second spatial light modulator 52 which registers a deflection which compensates for the deflection registered by the first spatial light modulator 51 so that elementary beams exit each with an optical axis perpendicular to the input face 41 of the fiber bundle 40. the bundles B _ls B _2, B ₃ are formed by light pulses that respectively have δίι delays δίι, OT ₂ and which, after passing through the single mode optical fibers F _ls F _2, F ₃ present the relative deviations of zero or reduced group velocity.

In the example of FIG. 2A, the elementary beams Bi at the output of the second spatial light modulator 52 are focused in a focal plane ₂ and an optical system 60 of the telescope type makes it possible to apply a magnification strictly less than 1 for adapting all the focusing points formed in the focal plane ₂ to the pattern formed by the fibers Fi at the input face 41 of the fiber bundle 40.

According to one variant, the focusing of the elementary beams Bi at the output of the second spatial light modulator 52 in the focal plane ₂ is ensured by means of the spatial light modulator 52 which introduces a parabolic phase at the level of each elementary beam Bi. Alternatively, the speed control device 50 may comprise at the output of the second spatial light modulator 52 an optics (not shown), for example a matrix of microlenses, which can ensure the focusing of each elementary beam.

The speed control device 50 as described by means of FIGS. 2A and 2B thus makes it possible in a simple manner to control the group speed at the level of each of the fibers. single mode optical fibers Fi of fiber package 40.

Of course, this speed control device, or GDC, can quite well be used to compensate for phase delays that have previously been characterized on the fibers of the fiber packet and / or to register at each elementary beam a phase function that will form the desired phase function at the distal end of the fiber packet 40, for example a parabolic function for the formation of a focus point.

In the example of FIG. 2A, these functions can be provided by one and / or the other of the first and second spatial light modulators 51, 52.

In the example of FIG. 2A, the first and / or the second spatial light modulator may be formed of a modulator based on segmented or diaphragm-shaped deformable mirrors, operating in reflection, or a liquid crystal matrix that may be operate in reflection or transmission.

FIGS. 3 to 6 show first experimental results obtained with an imaging system as described in FIG. 2A and making it possible to validate the method according to the present description.

In this example, the light source is a femtosecond laser, emitting pulses of 150 fs at a wavelength of 1.035 μιη. The pulse transport and control device comprises a single-mode optical fiber packet formed here of a multi-core fiber.

The multi-core fiber 40 used is illustrated in Figure 3A. It comprises a set of 169 monomode cores Fi arranged periodically and referenced from a central fiber F ₀ , as shown in Figure 3B. Each monomodal heart Fi is intended to receive at its proximal end an elementary beam Bi which passes through the heart to exit at a distal end, as explained above. The central core Fo forms the monomode reference fiber for the determination of the group delay Δχ; which characterizes each monomode heart Fi. The multi-core fiber also comprises in this example a multimode internal sheath 44 adapted to collect the light signal from the distal end to the proximal end. In the example shown in FIG. 3A, the inter-core distance is 1 1.8 μιη, the diameter of a mode in each monomode core is 3.6 μιη and the corresponding divergence of 0.12 radians; the diameter of the multimode inner sheath 44 is 250 μιη. The coupling measured between a monomode core Fi and its nearest neighbor is less than -25 dB, even with a curvature applied to the multi-core fiber of 12.5 cm. Ray.

A characterization of the relative group delays of each of the singlemode cores of the multi-core fiber 40 is carried out by means of a known method, for example a method described by means of FIGS. 9 and 10. FIG. 3C thus represents the delays of relative group measured Δχ; for hearts of index i of the multi-core fiber. The group delay is defined as the difference between the time that a light pulse takes to cross the fiber Fi and the time that an identical light pulse takes to cross the reference fiber F ₀ . Figure 3D shows the histogram of the set of group delay values.

As described by means of FIG. 2A, the speed control device 50 makes it possible to partition the N elementary beams intended to enter the single-core N cores of the multi-core fiber 40 into M groups on which M values will be printed. delay by means of M retardation blades P].

The M delay plates P] are for example formed by means of Ml sheets of glass of equal thickness, the index plate comprising j holes each capable of passing a group of elementary beams; the lamellas are stacked to form a delay plate comprising M zones for printing, on the elementary beams, M delays At _j . The holes can be made, for example, by laser ablation.

Thus FIG. 4 illustrates the production of 3 retardation plates Pi, P ₂ , P ₃ by means of two strips 56, 57 of substantially equal thicknesses, the strip 56 having 2 holes and the strip 57 having only one hole, the lamellae being arranged so as to form three areas defining the three delay plates and that will print respectively delays 0xôt _g, lxôt _g, 2xôt _g, where _g Ot is the delay introduced by passage of a pulse through a coverslip .

The delay blades can be formed also by any other known means. It may be for example M glass bars of equal diameter but different length. Each bar is likely to pass a group of elementary beams. The bars are for example arranged against each other, to print on the elementary beams, M delays _j . The length of a bar can be adjusted for example by polishing. Delayed slides can also be formed from a glass slide which is divided into M zones; by a micro-manufacturing method, each zone is hollowed to make M zones of different thickness. The micro-etching method can be dry etching (Reactive Ion Etching) or wet etching (HF) or use a focused ion beam (Focussed ion beam).

As for spatial light modulators, delay blades can work either in transmission or in reflection.

If we return to the example of FIGS. 3 to 6, each of the N elementary beams Bi will thus pass through one of the three delay plates Pi, P ₂ , P ₃ , as a function of the value of the delay of relative group Δχ; of the fiber Fi it is intended to cross. Since M is much smaller than N, a large number of elementary beams Bi are printed with the same delay in the intermediate focal plane.

Figures 5A and 5B show by histograms all the values of the relative group delays in a case where there is no group speed control device (FIG.5A) and in the case where the device Group Speed Control is present (FIG 5B). A clear reduction in the variance from one histogram to another is observed, and this already with 3 slides introducing 3 distinct values of delay.

Figure 6 shows the spatial pattern of a focal point at the output of the multi-core fiber with application of the method of control of the group speed (left) and without application of said method (right). In the figures below, the image of the focal point is represented, and in the upper figures, the spatial distribution of the intensity. Here again, these first experimental results show the gain in intensity obtained by the method according to the present description.

Figure 7 illustrates a schematic diagram of an example of an endoscopic lensless imaging system according to another example of the present disclosure.

This example is identical to that of FIG. 2A but represents the case of an aperiodic arrangement of monomode optical fibers in the fiber bundle 40. It is observed that the device and the method of transport and control of the pulses according to the present description are also applies to a fiber bundle having fibers arranged aperiodically.

Fig. 8A illustrates a diagram of an exemplary endoscopic lensless imaging system according to another example of the present disclosure.

In the diagram depicted in FIG. 8A, it is taken advantage of the fact that M "N first makes a differential delay between M parts of the collimated incident beam. In FIG. 8A, the collimated incident beam is subdivided into M = 2 under parts having a delay δίι or δ ₂ . This delay can be advantageously achieved by a micro-structured blade as described above. It is then a question of assigning, in the N fibers of the optical fiber packet, an elementary sub-beam with the chosen delay, here δίι or ôt ₂ . In this example, the first spatial light modulator 51 advantageously comprises a matrix of liquid crystals. We use for example, the additive property of the holograms which consists in generating, on M areas of the first spatial light modulator 51, a set of holograms making it possible to diffract the incident beam corresponding to the delay 6 in different directions. These different directions appear as focusing points in the plane of the second spatial light modulator 52 and the latter performs a deflection such that each elementary light beam penetrates perpendicularly to the input face of the optical fiber. The holograms formed at each of the M zones of the first spatial light modulator are for example computer generated holograms or "CGH" according to the abbreviation of the English expression "computer-generated hologram". Such holograms are described, for example, in Liesener et al. , "Multi-functional optical tweezers using computer-generated holograms", Opt. Commun., 185, 77 (2000).

FIG. 8B illustrates a diagram of an example of a lentil-free endoscopic imaging system similar to that of FIG. 8A but used in an application implementing pulses at two wavelengths, for example for applications in non-linear two-beam imaging.

According to this example, each fiber of the fiber packet 40 is intended to transport an elementary beam at a given wavelength and the relative group delay of this fiber is advantageously characterized at this wavelength. In this example, the first spatial light modulator 51 further allows the distribution of the elementary light beams formed of pulses at a given wavelength in an identified subset of the fibers of the fiber packet 40.

In the example illustrated in FIG. 8B, the beam at the first wavelength λι, materialized by simple arrows, thus passes for example through two delay plates Pi, P ₂ characterized by respective delays δίι and δ ₂ and the beam at the second wavelength λ ₂ , materialized by double arrows, passes through two delay plates P ₃ , P ₄ characterized by respective delays δ ₃ and δ ₄ . As in the example of FIG. 8A, the first spatial light modulator 51 makes it possible to form N elementary beams, each elementary beam of given wavelength being characterized by a delay introduced by the crossed blade and intended to enter a previously identified optical fiber of the fiber packet. For example, N / 2 fibers of the fiber packet receive elementary beams at the first wavelength λι, while the remaining N / 2 fibers of the fiber packet receive elementary beams at the second wavelength λ ₂ . In the example illustrated in FIG. 8B, six elementary beams are displayed, of which three at the wavelength λι and three at the wavelength λ ₂ . By for example, these two groups of fibers are chosen such that the transporting fibers λι and λ ₂ are intertwined on the proximal face of the fiber bundle. In the example illustrated in FIG. 8B, the interleaving is illustrated by the fact that, downstream of SLM2, the elementary beams alternate between λι and λ ₂ .

FIGS. 9 and 10 illustrate examples of methods for characterizing the relative group delays in a fiber packet 40 of a light pulse transport and control device according to the present description, for example for the characterization of a multi-core fiber. These methods are based on the known techniques of spectral interference (see, for example, Lepetit et al., "Linear techniques of phase measurement by femtosecond spectral interferometry for applications in spectroscopy", J. Opt.Soc.Am.B, 12 ( 12), 2467 (1995)). Figure 9 illustrates a method suitable for a distal measurement of group delays while Figure 10 illustrates a method suitable for proximal measurement of group delays, in which it is not necessary to have access to the distal end. of the fiber packet.

As illustrated in FIG. 9, the method for the characterization of group delays implements a fibered spectrometer 90 and a spatial light modulator 91. The measurement of the relative group delay Δχ; a fiber Fi, defined with respect to the travel time of a pulse propagating in a reference fiber F ₀ , comprises the following steps. Only the elementary beams Bi and B ₀ intended to enter the optical fibers Fi, F ₀ are formed. They pass through the optical fibers Fi and F ₀ respectively. Leaving the bundle of fibers 40 on the distal side, Bi and B ₀ diverge and overlaps spatially. In a plane where the recovery is almost total, an optical fiber 92 collects a portion of each beam. The optical fiber 92 conveys the collected light to the spectrum analyzer 90. The spectrum comprises a sinusoidal modulation (curves 94) whose period is equal to (Δχ;) ^-1 ; we thus deduce Δ¾, the desired value. In practice, to eliminate any background signal not coming from Bi or B ₀ , the spectrum is measured according to the principle of phase shift interferometry or the phase of Bi (with respect to Bo) is scanned using the modulator 91, according to the technique of phase shift interferometry (see, for example, Bruning et al., "Digital Wavefront Measuring Interferometer for Testing Optical Surfaces and Lenses", Appl. Opt. 13 (11), 2693 (1976). , Eqs. (3-6)).

It is also possible to measure Δ¾ without having access to the distal portion of the fiber bundle 40 as illustrated in FIG. 10. In fact, about 3% of Bo and Bi is reflected by the distal face of the fiber bundle, makes the difference in refractive index at the interface between the fiber bundle and the air. The reflected beams ΒΌ and B1 issuing from the proximal side of the fiber bundle 40 may be routed to an optical fiber 92 by means of a splitter plate 96 (for example a semi-reflective plate or a polarization separator cube). The measurement is done as described previously; in this case we measure (2Δχί) ^_1 because the pulses go back and forth in the fiber packet.

Although described through a number of detailed exemplary embodiments, the device for transporting and controlling light pulses for so-called "no lens" endo-microscopic imaging as well as endoscopic imaging systems and methods. Without a lens there are various variations, modifications and improvements which will become obvious to those skilled in the art, it being understood that these various variants, modifications and improvements are within the scope of the invention as defined by the claims which follow.

Claims

A device for transporting and controlling at least one wavelength light pulse for endo-microscopic lensless imaging comprising:

a packet (40) of N monomode optical fibers (Fi) arranged in a given pattern, for receiving a light beam formed of pulses (I ₀ ) at a proximal end and emitting a light beam at a distal end, each fiber single-mode optical (Fi) being characterized by a relative group delay value (Δχ;) defined with respect to the travel time of a pulse propagating in a reference monomode optical fiber (F ₀ ) of the fiber packet (40) ,

an optical group velocity control device (50) arranged on the proximal end side of the optical fiber packet and comprising:

A given number M of delay blades (P _j ), each blade allowing the introduction of a given delay (θ _j );

A first spatial light modulator (51) adapted to form from one or more incident light beams a number N of elementary light beams (Bi) each intended to enter one of said optical fibers (Fi), each elementary beam (Bi ) being intended to pass in a given retardation plate (P _j) such that the sum (OT _j + Δχ;) of the delay (OT _j) introduced by said delay plate and the relative group delay (Δχ;) of the optical fiber (Fi) for receiving said elementary light beam (Bi) is minimal in absolute value;

A second spatial light modulator (52) adapted to introduce on each of the N elementary light beams (Bi) a deflection such that each elementary light beam (Bi) enters the corresponding optical fiber (Fi) perpendicular to the input face of the optical fiber ;

a phase control device comprising programming means for one and / or the other of the first and second spatial light modulators (51, 52) for applying a phase shift to each of the elementary beams (Bi ) to register at the distal end of the fiber bundle (40) a determined phase function and / or to correct the phase variations introduced by each of the fibers of the fiber bundle (40).

Device for transporting and controlling light pulses according to claim 1, wherein the optical group velocity control device (50) comprises a first lens (53) and a second lens (54) forming an optical assembly with an intermediate focal plane (Σi) and wherein:

the delay blades (P _j ) are arranged in the intermediate focal plane (Σi) of the optical assembly;

the first spatial light modulator (51) is in an object focal plane of the first objective (53); and

the second spatial light modulator (52) is in an image focal plane of the second lens (54).

A light pulse transport and control device according to claim 1, wherein the optical group velocity control device (50) comprises a lens (58) and wherein:

the delay plates are arranged in a plane located upstream of the first spatial light modulator (51) and are adapted to form, from an incident beam formed by pulses (Io), M light beams, each light beam being formed of pulses characterized by a given group delay (δ _j );

the first spatial light modulator (51) is arranged in the object focal plane of the objective (58) and is intended to receive said M light beams;

the second spatial light modulator (52) is located in an image focal plane of the objective (58).

A light pulse transport and control device according to claim 3, wherein the first spatial light modulator (51) is formed of M holographic zones, each holographic zone being adapted to receive one of said light beams formed of pulses characterized by a group delay (ôt _j ) given.

A light pulse transport and control device as claimed in any one of the preceding claims, wherein the packet of N monomode optical fibers (Fi) is formed of a multi-core fiber.

A light pulse transport and control device as claimed in any one of the preceding claims, wherein the N monomode optical fibers are arranged aperiodically. Endo-microscopic imaging system comprising:

a source (10) of light pulses,

a device for transporting and controlling pulses emitted by said source according to any one of the preceding claims and

a light sensing path (20, 21) for traversing the monomode optical fiber packet (40) from its distal end to its proximal end.

A non-linear endo-microscopic imaging method without a lens by means of a packet of monomode optical fibers (40) arranged in a given pattern and each characterized by a relative group delay (Δχ;) defined with respect to the travel time of an impulse propagating in a reference monomode optical fiber (F ₀ ) of the fiber packet (40), the method comprising:

transmitting an incident beam formed of pulses (I ₀ ) to at least one wavelength on a first spatial light modulator (51) arranged in the object focal plane of a first objective (53) forming with a second objective (54) an optical assembly with an intermediate focal plane (Σi);

forming, by means of the first spatial light modulator (51) from the incident light beam, a number N of elementary light beams (Bi) each intended to enter one of said optical fibers (Fi), each elementary beam (Bi ) passing in a given delay plate (P _j ) characterized by a delay (ôt _j ) and arranged in the intermediate focal plane (Σi) of the optical assembly, such that the sum (ôt _j + Δχί) of the delay (ôt _j) ) introduced by said delay plate and the relative group delay (Δ¾) of the optical fiber (Fi) for receiving said elementary light beam (Bi) is minimal in absolute value;

the introduction by means of a second spatial light modulator (52) arranged in the image focal plane of the second objective (54) of a deflection on each of the N elementary light beams (Bi) such that each elementary light beam (Bi ) enters the corresponding optical fiber (Fi) perpendicularly to the input face of the optical fiber.

applying a phase shift on each of the elementary beams (Bi) by one or the other of the first and second spatial light modulators (51, 52) to register at the distal end of the fiber packet (40) a defined phase function and / or correcting the phase variations introduced by each of the fibers of the fiber bundle (40). A non-linear endo-microscopic imaging method without a lens by means of a packet of monomode optical fibers (40) arranged in a given pattern and each characterized by a relative group delay (Δχ;) defined with respect to the travel time of an impulse propagating in a reference monomode optical fiber (F ₀ ) of the fiber packet (40), the method comprising:

transmitting an incident beam formed by pulses (I ₀ ) at at least one wavelength and forming from said incident beam and by means of M delay plates (P _j ) each characterized by a delay ( OT _j) a number M of light beams, each of the M light beams being formed of pulses characterized by a group delay (OT _j) given,

forming by means of a first spatial light modulator (51) arranged in the object focal plane of a first lens (58) and from the M light beams of a number N of elementary light beams (Bi) for enter each in one of said optical fibers (fi), such that the sum (OT _j + Δ¾) of the delay (OT _j) of the light beam from which is formed the elementary light beam introduced (Bi) and the relative group delay ( Δχ;) of the optical fiber (Fi) intended to receive said elementary light beam (Bi) is minimal in absolute value;

the introduction by means of a second spatial light modulator (52) arranged in the image focal plane of the lens (58) of a deflection on each of the N elementary light beams (Bi) such that each elementary light beam ( Bi) enters the corresponding optical fiber (Fi) perpendicularly to the input face of the optical fiber - the application of a phase shift on each of the elementary beams (Bi) by one or the other of the spatial modulators light source (51, 52) for registering at the distal end of the fiber packet (40) a determined phase function and / or for correcting the phase variations introduced by each of the fibers of the fiber packet (40).

A non-linear endo-microscopic lensless imaging method according to any one of claims 8 to 9 comprising emitting at least two incident light beams, each incident light beam being formed of pulses at a distinct wavelength and wherein the first spatial light modulator (51) further enables the distribution of the elementary light beams formed of pulses at a given wavelength into a subset of the fibers of the fiber packet (40).